This invention relates generally to fiber optic connectors and, more specifically, to fiber optic connectors in which a plastic fiber end is formed by a melting process.
Plastic optical fibers used for data transmission are most often supplied in cable form in which the cable comprises a glass or plastic fiber core, a thin cladding, and a protective jacket which can include strengthening members. Connecting the fiber optic cable to another device, such as an electro-optic device or another cable, can be accomplished by two methods.
In one, bare fiber ends are contacted without any added terminal. This is a very delicate connection and is subject to damage of the connection, with attendant signal degradation across the interface.
The other method provides a terminal on the fiber end. This arrangement is very durable and provides a more reliable connection in systems requiring quick connect/disconnect with devices or other cables. The termination of the cable is often performed as a field operation, such as when installing data transmission systems, such as computer LANs (Local Area Networks).
To provide an optically efficient interface between an optical fiber and another device, it is necessary to mount the fiber end in a suitable terminal fitting in a manner that will provide good signal transmission across the interface. This requires that the fitting properly align the fiber with the terminal of other device. Such alignment is provided by mating surfaces formed on the fittings for the fiber and the device, which interfit to assure alignment.
An optically efficient interface also requires that the fiber have a flat, smooth end surface. Two methods of forming this end surface on plastic optic fibers during termination are in general usage. In both, the fiber end is exposed and a terminal is clamped or crimped onto the cable jacket, or the bare fiber is epoxied or otherwise cemented onto the terminal. The terminal can be a ferrule or a multiple fiber or combined fiber and electrical connector.
In one, the "polishing" method, the fiber end is snipped off to form a generally flat surface near the terminal face of the end fitting. This surface is then polished to a predetermined degree of smoothness to eliminate pits which adversely affect light transmission. This polishing method is a laborious, exacting and, thus costly, process which requires repeated visual inspection to determine when the predetermined degree of smoothness is achieved.
The second "hot plate" method involves stripping the cable jacket, inserting the fiber into an end fitting and projecting it through the exit aperture beyond the terminal face of the fitting. The fiber end is then snipped off a predetermined short distance beyond the terminal face, a smooth plate is brought into contact with the fiber end, heat is applied to the plate to melt the fiber, the heat is removed from the plate, the fiber cools and solidifies, and the plate is removed. This "hot plate" method leaves the fiber end with the same smooth, flat surface as the plate.
One problem with the hot plate method is disposal of the excess reflowed fiber material, which remains after melting, in a manner that does not adversely affect light transmission through the terminal. Several methods are in current usage. In one, the excess reflowed material resulting from the melting is formed into a smooth-surfaced "pancake" lying atop the fitting's terminal face. While this provides the requisite smooth, flat surface, this pancake protrudes from the surface and is unbounded, which allows light leakage and degrades light transmission to an unacceptable degree.
Another problem is caused by the thickness of this pancake. Since it is uncontrolled, it prevents precise mating of the terminal with the other device. This affects alignment of the fiber with the other device and light transmission across the interface.
A further problem is contamination of the fiber material by the cladding material, caused by their intermixing during melting. This further degrades light transmission.
Two design modifications of the end fitting have been made in an effort to overcome the problems caused by the protruding, unbounded pancake. In one, the thickness of the pancake is reduced by flaring the exit aperture with a conical countersink. If the excess reflowed material exceeds the capacity of this slight enlargement, a protruding pancake is formed, with the same problems just described.
If melting produces insufficient excess reflowed material to completely fill the flared enlargement, the hot plate will make incomplete contact and form a pitted fiber end surface, resulting in seriously degraded light transmission. In either event, the cladding material will again contaminate the core material during melting, with the resulting adverse consequences noted above.
In the other modification, the protruding pancake is eliminated by moving it subsurface. The exit aperture is countersunk to form an enlarged cylindrical recess in the fitting terminal face. Upon melting, the pancake is formed completely within the recess. A smooth surface may be formed by the hot plate, regardless of the quantity of the excess reflowed material. The mating and alignment problems caused by the protruding pancake lying atop the terminal face of the fitting are eliminated.
However, a shortage of material (i.e. insufficient to fill the recess) will result in an irregular boundary, resulting in signal degradation. Although the enlarged recess prevents formation of a protrusion, it still can cause light leakage due to the lack of a defined boundary if the reflowed plastic material does not reach the recess walls.
The hot plate melting results in a gap between the melted fiber end surface (top of the pancake) and the point of fiber entrance to the recess if the recess is not exactly filled, which it never is. In order to guarantee that the pancake does not protrude above the terminal face, this recess must be sufficiently voluminous to accommodate the largest quantity of excess reflowed material that could occur. Thus the gap is inevitable. In any event the ideal light guiding properties of the fiber are lost at the entrance to the recess where the irregular pancake of melted material begins.
When the fitting is mated with the other device, the gap introduces a new source of light leakage and signal degradation. Also, the problem of contamination by the intermixed cladding material persists.
All of the currently-used methods and apparatus for terminating an optical fiber have inherent physical problems that require extremely tight processing tolerances to minimize degradation of light transmission across the terminal interface in a connection. A slight variance of excess melted fiber material will cause light leakage that results in signal degradation and consequent inferior light transmission at the terminal interface.
Thus, a need exists for an optical fiber termination method and apparatus which eliminates the problems causing light leakage and resultant signal degradation.